Systems and methods for cooling electronics components employing vapor compression refrigeration with selected portions of expansion structures coated with polytetrafluorethylene

ABSTRACT

Systems and Methods of cooling heat generating electronics components are provided employing vapor compression refrigeration. In one embodiment, the vapor compression refrigeration system includes a condenser, at least one expansion structure, at least one evaporator, and a compressor coupled in fluid communication to define a refrigerant flow path, and allow the flow of refrigerant therethrough. The at least one evaporator is coupled to the at least one heat generating electronics component to facilitate removal of heat produced by the electronics component. At least a portion of the at least one expansion structure is coated with a polytetrafluorethylene in the refrigerant flow path for inhibiting accumulation of material thereon. The polytetrafluorethylene coating has a thickness sufficient to inhibit accumulation of material in a pressure drop area of the expansion structure without significantly changing a pressure drop characteristic of the pressure drop area.

TECHNICAL FIELD

The present invention relates generally to heat transfer mechanisms, andmore particularly, to cooling systems and methods for removing heatgenerated by one or more heat generating electronics components. Moreparticularly, the present invention relates to cooling systems andmethods employing vapor compression refrigeration.

BACKGROUND OF THE INVENTION

As is known, operating electronic devices produce heat. This heat shouldbe removed from the devices in order to maintain device junctiontemperatures within desirable limits. Failure to remove produced heatresults in increased device temperatures, potentially leading to thermalrunaway conditions. Several trends in the electronics industry havecombined to increase the importance of thermal management, includingheat removal for electronics devices, particularly in technologies wherethermal management has traditionally been less of a concern, such asCMOS. In particular, the need for faster and more densely packedcircuits has had a direct impact on the importance of thermalmanagement. First, power dissipation, and therefore heat production,increases as device operating frequencies increase. Second, increasedoperating frequencies may be possible at lower device junctiontemperatures. Further, as more and more devices are packed onto a singlechip, power density (Watts/cm²) increases, resulting in the need toremove more power from a given size chip or module. Additionally, acommon packaging configuration for many large computer systems today isa multi-drawer rack, with each drawer containing one or more processormodules along with associated electronics, such as memory, power andhard drive devices. These drawers are removable units so that in theevent of failure of an individual drawer, the drawer may be removed andreplaced in the field. A problem with this configuration is the increasein heat flux at the electronics drawer level. The above-noted trendshave combined to create applications where it is no longer desirable toremove heat from modem devices solely by traditional air coolingmethods, such as by using traditional air cooled heat sinks. Thesetrends are likely to continue, furthering the need for alternatives totraditional air cooling methods.

One approach to avoiding the limitations of traditional air cooling isto use a cooling liquid. As is known, different liquids providedifferent cooling characteristics. For example, refrigerants or otherdielectric fluids (e.g., fluorocarbon fluid) may have an advantage inthat they may be placed in direct physical contact with electronicdevices and interconnects without adverse affects such as corrosion orelectrical short circuits. For example, U.S. Pat No. 6,052,284, entitled“Printed Circuit Board with Electronic Devices Mounted Thereon”,describes an apparatus in which a dielectric liquid flows over andaround several operating electronic devices, thereby removing heat fromthe devices. Similar approaches are disclosed in U.S. Pat. No.5,655,290, entitled “Method for Making a Three-Dimensional MultichipModule” and U.S. Pat. No. 4,888,663, entitled “Cooling System forElectronic Assembly”.

Notwithstanding the above, there remains a large and significant need toprovide further useful cooling system enhancements for facilitatingcooling of heat generating electronics components, such as one or moreelectronics modules disposed, e.g., in a book of an electronics rack ofa computer installation.

SUMMARY OF THE INVENTION

In vapor compression refrigeration systems employed for cooling one ormore heat generating electronics components, it has been discovered thatmaterial can agglomerate in certain pressure drop areas of expansionstructures within the vapor compression refrigeration system. Duringrefrigerant/oil transport through a hot compressor, any long-chainmolecules and other typically non-soluble compounds at room temperaturecan go into solution in the hot mixture. These, as well as otherphysically transported impurities, then fall out of solution when therefrigerant/oil cools down. A layer of “waxy” material can build up inthe pressure drop areas and act as a sticky substance which then catchesother impurities. This material has been found to amass on expansionstructures such as expansion valves, and particularly on the pin andorifice control region in the refrigerant flow path of the expansionvalve. This amassing of material can interfere with the normal controlvolumes and interfere with the control of motor steps (due tounpredictable valve characteristic changes). This is particularly truewhen the vapor compression refrigeration system is employed in a coolingapplication for removing heat from a heat generating electronicscomponent as described herein since control of the valve in thisenvironment is a very sensitive application and expansion structuregeometries are typically very small. To eliminate all contaminants fromthe vapor compression refrigeration system would be too costly, if notimpossible. Thus, presented herein is a solution based on coating onlyselected pressure drop areas of the vapor compression refrigerationsystem to eliminate or reduce the clogging effect of debris andimpurities in critically tight areas. This application is particularlysignificant in a cooling system where little of the expansion valve'savailable valve volume is employed during a vapor compression cycle.

The shortcomings of the prior art and additional advantages are providedthrough the provision of a cooling system for cooling at least one heatgenerating electronics component. The cooling system includes a vaporcompression refrigeration system. The vapor compression refrigerationsystem has a condenser, at least one expansion structure, at least oneevaporator and a compressor all coupled in fluid communication to definea refrigerant flow path and allow the flow of refrigerant therethrough.The at least one evaporator facilitates removal of heat produced by theat least one heat generating electronics component, while at least aportion of the at least one expansion structure is coated with apolytetrafluorethylene in the refrigerant flow path. Thepolytetrafluorethylene coating inhibits accumulation of material onselected pressure drop surfaces of the at least one expansion structure.

In another embodiment, a vapor compression refrigeration cooling systemis provided for cooling at least one heat generating electronicscomponent. This cooling system includes: a condenser, a firstelectrically controlled expansion valve coupled to the condenser, afirst evaporator coupled to the first electrically controlled expansionvalve; a second electrically controlled expansion valve coupled to thecondenser, a second evaporator coupled to the second electricallycontrolled expansion valve; a controller providing control signals tothe first electrically controlled expansion valve and the secondelectrically controlled expansion valve to control operation of thefirst electrically controlled expansion valve and the secondelectrically controlled expansion valve; and a compressor coupled to thefirst evaporator, the second evaporator and the condenser. Thecondenser, the first electrically controlled expansion valve, the firstevaporator, the second electrically controlled expansion valve, thesecond evaporator, and the compressor are coupled in fluid communicationto define multiple refrigerant flow paths, each refrigerant flow pathallowing flow of refrigerant therethrough. The first evaporator and thesecond evaporator facilitate removal of heat produced by the at leastone heat generating electronics component. At least a portion of thefirst electrically controlled expansion valve and at least a portion ofthe second electrically controlled expansion valve are coated with apolytetrafluorethylene in the respective refrigerant flow paths forinhibiting accumulation of material thereon.

In a further aspect, a method of fabricating a vapor compressionrefrigeration system for cooling at least one heat generatingelectronics component is provided. The method includes: (i) providing acondenser, at least one expansion structure, at least one evaporator,and a compressor; (ii) providing a polytetrafluorethylene coating on atleast a portion of the at least one expansion structure; (iii) couplingthe condenser, at least one expansion structure, at least one evaporatorand compressor in fluid communication to define a refrigerant flow path;and (iv) providing refrigerant within the refrigerant flow path of thevapor compression refrigeration system to allow for cooling of the atleast one heat generating electronics component employing sequentialvapor compression cycles, wherein the polytetrafluorethylene coating isprovided on the at least a portion of the at least one expansionstructure in the refrigerant flow path for inhibiting the accumulationof material thereon.

Further, additional features and advantages are realized through thetechniques of the present invention. Other embodiments and aspects ofthe invention are described in detail herein and are considered a partof the claimed invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter which is regarded as the invention is particularlypointed out and distinctly claimed in the claims at the conclusion ofthe specification. The foregoing and other objects, features, andadvantages of the invention are apparent from the following detaileddescription taken in conjunction with the accompanying drawings inwhich:

FIG. 1 depicts one embodiment of a cooling system comprising a vaporcompression refrigeration system, in accordance with an aspect of thepresent invention;

FIG. 2 illustrates one example of a flowchart that shows how a ModularRefrigeration Unit (MRU) code which contains a method to monitor andregulate multi-chip module (MCM) temperature under primary MRU cooling,a power control code (PCC) which contains a method to determine andcommunicate the thermal state or range that equates to a specifictemperature and voltage condition, and a Cycle Steering Application(CSA) code which contains a method of matching the various logic clocksto the thermal degrade states that exist, may interact in a singletemperature-power-logic control system, in accordance with an aspect ofthe present invention;

FIG. 3 depicts a system schematic where the MRU code, PCC code, and CSAcode are physically located in a server having four processor books ornodes, cooled in primary mode by two MRUs, and in back-up mode byblowers, in accordance with an aspect of the present invention;

FIG. 4 is a cross-sectional, elevational view of one embodiment of anexpansion structure comprising an expansion valve having an expansionpin and an expansion orifice which are part of a refrigerant flow pathof a vapor compression refrigeration cooling system, in accordance withan aspect of the present invention;

FIG. 5 is an enlarged, cross-sectional view of the expansion orifice andexpansion pin illustrated in FIG. 4, in accordance with an aspect of thepresent invention;

FIG. 6 is an isometric view of one embodiment of an expansion pin for ofan expansion valve of a vapor compression refrigeration system, whereinmaterial/debris is shown amassed on exposed surfaces of the expansionpin, which would be in a pressure drop area of the refrigerant flow path(not shown);

FIG. 7 is an isometric view of an expansion pin of an expansion valve ofa vapor compression refrigeration system, wherein the expansion pin iscoated with a layer of polytetrafluorethylene in pressure drop areas therefrigerant flow path, in accordance with an aspect of the presentinvention; and

FIG. 8 is a cross-sectional, elevational view of an expansion orificeand expansion pin of an expansion valve of a vapor compressionrefrigeration system showing selected pressure drop areas of theexpansion pin and inner surface of the expansion orifice coated with apolytetrafluorethylene, in accordance with an aspect of the presentinvention.

BEST MODE FOR CARRYING OUT THE INVENTION

As used herein, the term “electronics rack” includes any frame, rack,blade server system, etc., having at least one heat generatingelectronics component of a computer system or electronics system, andmay be, for example, a stand alone computer processor having high, midor low end processing capability. In one embodiment, an electronics rackmay comprise multiple books, each book having one or more heatgenerating electronics components requiring cooling. Each “heatgenerating electronics component” may comprise an electronic device, anelectronics module, an integrated circuit chip, a multi-chip module,etc. An “expansion structure” is any structure or area in a vaporcompression refrigeration system where there is a pressure drop, andthus refrigerant expansion during a refrigerant compression/expansioncycle. As used herein, the term “expansion structure” includes anystructure of a pressure drop area and adjacent areas where anagglomeration would effect an expansion structure characteristic,including any thermally effected conduction zones and any downstreammass transport zones. Examples of expansion structures include expansionvalves, including electronic expansion valves, thermal expansion valves,hot-gas bypass valves, or mechanical expansion valves, as well as otherrefrigerant expansion structures such as a fixed expansion orifice in anevaporator. As used herein, an “expansion orifice” means any openingdefined by a component within the vapor compression refrigerationsystem, and includes a fixed orifice in an evaporator, as well as anopening defined by an inner surface of an expansion valve. Further, theword “refrigerant” is used herein to refer to any coolant which can beemployed in a vapor compression/expansion system.

One example of refrigerant within a cooling system in accordance with anaspect of the present invention is R-134A coolant (i.e., 1,1,1,2tetrafluoroethane), however, the concepts disclosed herein are readilyapplied to other types of refrigerants, other dielectric fluids (e.g.,fluorocarbon fluid), or other types of coolants while still maintainingthe advantages and unique features of the present invention.

FIG. 1 depicts a cooling system 100 as an exemplary embodiment of thepresent invention. Cooling system 100 includes a condenser 104 and twoevaporators 106 and 108. Evaporators 106 and 108 cool heat generatingelectronics components 110 and 112, respectively. In this embodiment,components 110 and 112 are multi-chip modules (MCMs), but it isunderstood that other components (e.g., single processors, memory) maybe similarly cooled.

Both evaporators 106 and 108 are supplied refrigerant from a commoncondenser 104. An expansion valve 114 receives high pressure liquidrefrigerant from condenser 104 and generates low pressure liquidrefrigerant to evaporator 106. An expansion valve 116 receives highpressure liquid refrigerant from condenser 104 and generates lowpressure liquid refrigerant to evaporator 108. Expansion valves 114 and116 are electrically controllable. A controller 120 provides controlsignals to expansion valve 114 and expansion valve 116 to controlrefrigerant flow and pressure drop across each expansion valve. In anexemplary embodiment, expansion valves 114 and 116 each includes astepper motor that responds to control signals from the controller 120.The stepper motor opens or closes an orifice in the expansion valve toregulate refrigerant flow and pressure drop. Controller 120 executes acomputer program to control the expansion valves 114 and 116.

The low pressure liquid refrigerant exits the expansion valves 114 and116 and is supplied to evaporators 106 and 108, respectively. Therefrigerant in each evaporator 106 and 108 is converted to low pressurevapor refrigerant, in part, though further fixed expansion structures107, 109, respectively, and provided to a common compressor 122. Highpressure vapor from compressor 122 is supplied to condenser 104. Fan 126establishes air flow across condenser 104 to facilitate cooling the highpressure vapor refrigerant to high pressure liquid refrigerant.

A plurality of temperature sensors are distributed throughout thecooling system 100. The sensors may be thermistors or other knowntemperature sensors. Sensor T1 measures air temperature enteringcondenser 104. Sensor T2 measures air temperature exiting condenser 104.Sensors T3 and T3′ provide redundant measurement of refrigeranttemperature exiting condenser 104. Sensor T4 measures refrigeranttemperature entering condenser 104. Sensor T6 measures refrigeranttemperature entering evaporator 106 and sensor T7 measures refrigeranttemperature exiting evaporator 106. Sensor T8 measures refrigeranttemperature entering evaporator 108 and sensor T9 measures refrigeranttemperature exiting evaporator 108. Sensor T_(hat1) measures temperatureat electronics component 110 and sensor T_(hat2) measures temperature atelectronics component 112.

Each temperature sensor generators a temperature signal which issupplied to controller 120 and shown as T_(in). The control 120 adjustthe expansion valves 114 and/or 116 in response to one or more of thetemperature signals to maintain the logic modules 110 and 112 at apredefined temperature. Controller 120 controls expansion valves 114and/or 116 to obtain desired superheat valves while maintaining eachelectronics component at a desired temperature. Each component 110 and112 may be maintained at a different temperature or the sametemperature, even if each component has different heat loads.

Evaporators 106 and 108 may be connected to the refrigerant supply andrefrigerant return lines through quick disconnect connectors 130. Thecontrollable expansion valves 114 and 116 allow an evaporator to beremoved for maintenance or upgrade while the other evaporator, condenserand compressor continue to operate. For example, expansion valve 114 canbe closed and the refrigerant from evaporator 106 removed by the suctionof the compressor 122. Evaporator 106 can then be removed for service,upgrade, etc.

Although two evaporators are shown connected to one modularrefrigeration unit (MRU) (condenser, compressor, expansion valves andcontroller), it is understood that more than two evaporators may becoupled to each MRU.

In an exemplary embodiment in accordance with the present invention, oneembodiment joins methods to monitor and control the temperatures ofelectronics components 110, 112, to report the temperature state and toadjust the voltage levels appropriately and to adjust the various clockspeeds which govern CMOS circuits that are effected by the change intemperature and/or voltage.

A detailed description of one method of monitoring and controlling thetemperature of a hybrid cooling system 100 is described below withreference to FIGS. 2 and 3. FIG. 2 illustrates a flowchart that showshow a Modular Refrigerant Unit (MRU) code 200, which contains a methodto monitor and regulate the MCM (i.e., one example of a component)temperature under primary MRU cooling, interfaces with a Power ControlCode (PCC) 210, which contains a method to determine and communicate thethermal state or range that equates to a specific temperature andvoltage condition of each MCM, and a Cycle Steering Application (CSA)code 220, which contains the method of matching the various logic clocksto the thermal degrade state that exist. The MRU code, PCC code and CSAcode, all interact into a single temperature-power-logic control systemgenerally indicated as 230.

FIG. 3 shows one embodiment of a system schematic wherein the MRU code200, the PCC code 210 and the CSA code 220 are physically located in aserver that has four Processor (PU) books or nodes 242, 244, 246, 248,respectively, each having an electronics component or MCM cooled inprimary mode by one of two MRUs 250, 252 and in backup mode by twoblowers 254. The backup blowers 254 provide air cooling of all PU books242, 244, 246, 248, for MRU failures or light logic load state. Each MCMis operably connected to a main system board generally indicated at 256.The MRU code 200 is in each MRU 250, 252. The PCC code 210 is splitbetween Base Power Cage Controllers or Base Power Assembly 260, 262 anddigital converter assemblies (DCA) cage controllers (DCA 01, 02, 11, 12,21, 22, 31, 32). The Base Power Assembly 260, 262 provides high voltageDC power to the entire server 240 and the DCA converts the high DC powerto low DC voltages used by each circuit. The CSA code 220 is located inthe first Processor book 244 (labeled PU Book 0) of multi-node server240.

Each MCM (not shown) in each PU book 242-248 includes a hat 274 inoperable communication with a cooling unit 10 and connected to a thermalsensor assembly 276. Each thermal sensor assembly 276 preferablyincludes three thermistors configured to sense a temperature of acorresponding MCM.

The thermal sensors are compared for miscompare properties and forinsanity limits to make sure the temperatures measured are accurate. Onesensor is directly sensed by the Modular Refrigeration Unit (MRU)indicated generally at 278 and the other two are read by the powersupply feeding the MCM power indicated generally at 280 to insure fullredundancy and accuracy of this reading. The MRU reads an MCM hatthermistor sensor directly through its drive card to enable continualmonitoring and thermal regulation in case of a cage controller (cc)failover. MCM hat thermistors that are read by each DCA power supply aswell as by the MRU are compared to each other by the MRU and PowerControl Code to identify any faulty sensors and eliminate the faultysensors from consideration generally indicated at 286 in FIG. 2. Thisinsures redundancy of control and cooling status function. The powersupply thermistor also serves for thermal protection of the MCMs,dropping power if the temperatures are near damage limits.

The control of the primary cooling system is done by using aProportional Integral Derivative (PID) control loop of an electronicexpansion valve to each evaporator as described with reference to FIG. 1and generally indicated at 290 in FIG. 2. The PID control loop regulatesthe coolant flow to each MCM being cooled. The coolant flow is increasedby opening the electronic expansion valve if the MCM is too warm or ishigher than targeted and the flow is reduced by closing the valveposition if the MCM is too cold or cooler than targeted.

When the PID control has opened its electronic expansion valve to thefully open position providing maximum coolant to a given MCM, thecompressor speed then executes its own PID control loop to deliveradditional cooling capacity to the MCM. In other words, a second PIDcontrol loop controls the compressor speed if the valve regulating theflow of coolant to a respective evaporator has reached its maximumcooling position.

Similarly, the blower speed of blower 126 cooling the refrigerantcondenser 104 is controlled by the cooling capacity needs from the MRU.More specifically, blower speed controls provide more air for coolingthe MRU condenser 104 when the thermistors T1 and T2 on the condenser103 and ambient air indicate that inadequate condensing is taking place.Also, the speed of condenser blower 126 is increased in a warm ambient.

MCM power data 284, read by the Power Control Code 210 and provided tothe MRU code 200 every 2.5 seconds, determines if a given MCM no longerhas its clocks functioning. If the MCM power stays low, indicating anon-functional Processor book, for sufficient time, the refrigerantcoolant supply is stopped by completely closing the expansion valve tothat MCM only and turning on the backup blowers 254 at a reduced speed.In this manner, other MCMs in the same server can stay refrigerantcooled while the MCM that has check stopped or otherwise ceased tofunction logically will be air cooled. Refrigerant cooling and MCMwithout adequate logic power can lead to condensation forming on itsexternal surfaces. For example, when regulating light heat loads to afixed temperature, the expansion device must significantly close therefrigerant flow rate, which lowers the pressure and hence therefrigerant temperature inside the evaporator cooling the MCM. When theclocks are off, the expansion valve closes so far that the evaporatorpressure may be sub-atmospheric, which creates very cold localtemperatures. These cold local temperatures with low heat flux andoutside regions of the MCM can get cold enough to form condensate afterextended operation in this condition.

The MRU code 200 also provides a function that enables virtually all ofthe refrigerant to be removed from the evaporator of a correspondingcooling unit before the refrigerant lines are opened for servicing theMCM or cooling hardware, as discussed above with respect to FIG. 1. Thisis provided by closing the electronic expansion valves for some periodbefore turning off the compressor, resulting in a partial vacuum thatremoves the refrigerant from the evaporator and connecting hoses, Thebenefits include better ecology and consistent refrigerant charge beforeand after the MRU is reconnected.

This temperature control code, together with primary and/or secondarycooling hardware, has the ability to program and run the MCMs atdifferent or “biased” conditions to enable the MCM to be tested beyondthe normal temperature conditions it sees in actual use. The temperaturebias testing may be done while the logic voltage is also biased. In theprior art, these bias cooling functions required special tester coolinghardware and test code which was costly and inefficient compared tocombining this stress test thermal function in the actual coolingsystem. Secondary cooling uses a PID loop also to achieve MCMtemperature target that may be outside of the normal operating range.

Still referring to FIGS. 2 and 3, a detailed description of the PowerContol Code (PCC) 210 which principally includes a method for monitoringthe actual thermal or degrade state and for making suitable power andcooling adjustments, as well as reporting this state to the CSA code220, follows below. The thermal states of each MCM are monitored and thestate of each MCM is communicated to a function that determines theproper clock cycle time, called the Cycle Steering Application (CSA)code 220. This function tells the CSA code 220 both which cycle timerange of the circuits are now operating in and whether the cause of thefailure of the primary cooling means has been repaired or not.

In particular, PCC 210 continually monitors and posts “cooling state”data to the CSA code 220 indicated generally as 292. The thermal stateis defined by discrete temperature ranges that are associated with agiven clock speed as the proper speed to operate. In other words, thefull operating temperature range from coldest to ambient to shut-downfor thermal protection is subdivided into smaller discrete operatingranges. The coldest steady state temperature range is called the normalstate, and is the temperature range kept under normal primary coolingmeans (e.g., MRUs 250, 252 and cooling units 10). When the primarycooling means no longer functions properly, the cooling state, sensedvia the MCM sensors 276, is reported as a specific “degrade state”. Byway of example, there may be between 2 and 4 degrade states betweennormal operation and thermal shut-down, but more or less are alsocontemplated, and hence, these concepts are not limited to between 2 and4. Within a given degrade state, there exists one “optimum” set of clockspeeds.

The PCC 210 reads the actual current 294 and voltage 284 being suppliedto each MCM as well as its temperature 286. Based on the leakagecharacteristics of the CMOS technology, the capacity left in the powersupply providing the current to the MCM, and operating temperatures, thePCC 210 may either increase or decrease or leave alone the appliedvoltage level to each set of circuits indicated generally 296.

When the voltage is increased, the increased voltage enables a higherrange of operating temperatures before a given degrade state isindicated to the CSA code 220 to slow the clocks. Hence, the highervoltage can delay the need to operate in a slower clock range. This isbecause CMOS switches faster at higher voltages somewhat offsetting theslowing effects of warmer circuits.

Normally, it is desirable to increase voltage applied to the circuits tooffset some of the slowing effect on circuit switching of warmercircuits. Typically, a 6% increase in voltage will cause circuits toswitch about 4% faster, offsetting a 25° C. temperature rise. However,with recent circuit technology, power increases strongly with highertemperature and increased voltage. In some cases it may require thevoltage to be dropped when the junction temperature rises significantly,even though this lowering of voltage will increase the amount of slowingof the clock frequency that is needed. This disclosure includes allthree voltage responses to loss of normal cooling: doing nothing,increasing voltage, and lowering voltage. A voltage alteration may bedone to all components in a system or just to specific electronicscomponents that are exceeding normal cooling limits.

Under circumstances where additional leakage currents due to hotter CMOScircuit temperatures cause concern of either heating the MCM beyond itssafe operating temperature range or requires additional current than theDCAs are able to provide, the PCC 210 lowers the voltage applied to theCMOS circuits when a temperature degradation occurs. The effect on the“cooling degrade state” is to hasten its arrival as the combination oflower voltage and warmer circuits requires faster clock speedadjustments.

The PCC 210 takes into account both the MCM temperatures and appliedvoltage when it notifies the CSA code 220 of a change in “coolingstate”. The PCC 210 continually monitors the MCM thermistors 276 andprovides the MRU with information if a sensor value is erroneous as wellas the actual good values.

The PCC 210 sends the message to the CSA code 220 when the first degradestate is reached, indicating that the primary cooling system is notfunctioning normally. When it has been determined that this degradestate is due to a failure of the cooling hardware, the PCC 210 sets afault flag for the primary cooling system, which is not removed untilthe primary cooling system is repaired. The PCC 210 posts this interruptto the CSA code 220.

The PCC 210 automatically turns on the backup cooling blowers or coolingfans 254 if the temperatures are above acceptable levels for the primarycooling system. The fan speeds are controlled in such a manner that theMCM temperature will not oscillate between cooling states unless theroom ambient also oscillates.

The PCC 210 turns on the backup cooling blowers 254 at a speed toprovide a temperature sufficiently above the temperature the firstdegrade state occurred so as to prevent “cooling state oscillation” whenthe backup blowers 254 are first turned on generally indicated at 298.Steady state air cooling mode will be in degrade one or a slower degradestate, but if the backup blowers 254 are turned on immediately after thefirst degrade state is posted, then the additional backup cooling maycause a temporary spike down into the normal range temperature only tobe soon followed by revisiting the first degrade state. It will berecognized by one skilled in the pertinent art that it is advantageousto minimize the occurrences of changing degrade states.

The PCC 210 continually samples the current and voltage being used byeach MCM and communicates this power data to the MRU code as MCM powersstate 284. The PCC 210 also suitably adjust the power supply voltagelevels at 296 being applied to the circuits. Raising the voltages willoffset some of the speed lost by higher operating temperatures for someservers still operating in a safe temperature range and with extra poweravailable from the power supply. For an MCM within server 240 which isoperating near its upper temperature limit or for which the power supplyhas no additional current to supply, the PCC 210 either leaves thevoltage unchanged or lowers it to reduce leakage currents in CMOScircuits. Hence, by sensing MCM temperatures and current being used bythe MCM, the PCC 210 determines what if any voltage adjustment issuitable.

At all times, the existing temperatures and voltage conditions togetherdefine a suitable “thermal state” or range within which a specific setof clock speeds is optimum. The PCC 210 notifies the CSA code 220 of theproper speed range or “thermal state” that the MCMS are operating in atall times at 292. This speed range may also be called a degrade state asdescribed above.

The PCC 210 maintains a cooling state for each MCM available for the CSAcode 220 to monitor at any time. The PCC 210 also provides periodicredundancy checks to insure that the backup blowers 254 are operatingproperly. When a primary cooling source having a fault, such as an MRU,is repaired, the PCC 210 clears defect status registers set which arevisible to the CSA code 220. Likewise, the PCC 210 also sends aninterrupt to the CSA code 220 if the primary cooling system, e.g., MRUs250, 252, needs service.

The Cycle Steering Application (CSA) code 220 provides a fail-safemethod of adjusting the clock speeds in an optimum manner when thecooling state changes. This method of clock speed adjustment includesdetermining if a cooling failure has been repaired prior to increasingthe clock speeds to prevent oscillating clock speeds. It should be notedthat the clock speed follows the temperature and voltage conditions atall times. Further, the time from a change of circuit temperature to acorresponding change in clock speed is slow enough that the temperaturesof the circuits change minimally, less than about 1° C., during thisprocess.

The CSA node 220 includes an interrupt handler that reads directly fromthe PCC 210 the cooling state of each MCM as well as receivinginterrupts on these states.

For systems with multiple processor books or nodes, the CSA code 220determined which MCM has the slowest cooling state. This is the statethat governs the safe clock speed of the system indicated generally at310 in FIG. 2. The multiple clock boundaries on multiple oscillatorswith predefined ratios are always maintained.

The CSA code 220 determines if any cooling defective hardware registersare set whenever a cooling state is increased calling for a faster clockspeed. If the hardware defect register is set, it means the cause of thecooling degradation has not yet been fixed and the change in coolingstate is likely due to transient change in ambient or other transientconditions. Hence, the server clock speeds are not re-adjusted fasteruntil the defective cooling hardware is replaced and the registercleared. This is true even after the machine is re-initial microcodeloaded (reIMLed) or rebooted. If there is uncertainty in the coolingstate due to communication problems, the slowest, safest cooling stateis employed by the CSA code 220.

When the CSA code 220 determines it is appropriate to make a change inseveral clock speeds, it alters the phase lock loops (PLL) on the clocksynthesizers in a sequence of very small steps until its new targetedclock speed is reached generally indicated as 312. The phase lock loopsare stepwise changes always retaining the optimum operating ratiobetween the various clocks that may be affected. The steps aresufficiently small to pose no risk to proper operation due to change inclock ratios during this adjustment process.

Every step is performed in a two step commit algorithm, e.g., thecurrent step and the next step PLL values are saved in a persistentstorage concept made up by using SEEPROMS residing on the current andbackup cage controller 262, 262. After the change is written to the PLLand read back for verification, the saved current value is updated. Thisis done to provide protection in case a speed change is interrupted by acage controller switchover.

The width of the small steps taken on the phase lock loops is less thanthe normal jitter of the phase lock loop normal output. This allows thestep variation not to be detected by the target clock receivingcircuitry. In this manner, all of the affected clocks are stepped insmall increments until the targeted clock speed is achieved.

The PLLs are on two oscillator cards 263, one in charge, one in backupmode. At all times the optimum ratio between clocks is maintained as thephase lock loops are moved in minimal increments or decrements.

Prior to power good time, the CSA code 220 issues a “Pre-Cooling”command to insure that the MCM temperatures are in proper normal stateprior to turning on the clocks. This also prevents a sudden surge ofpower from the CMOS logic beginning to switch. Without pre-cool, thiscould cause a quick degrade state to occur because the refrigerantsystem takes some time to get its cooling cycle established. Whenpre-cooled state is reached the PCC 210 notifies the CSA code 220 of thesame and IML is initiated.

The PLLs are initially loaded with a pattern, which is hard wired on thecards and loaded in parallel at power good time. Normally, PLLs areloaded serially, but this is exposed to shift errors which would lead towrong clock speed settings.

The exact process of initializing clocks includes first verifying theright oscillator card 263. Then, the pattern matching the actual systemspeed is loaded into the line drivers and read back to insure that thereare no errors or hardware failures. Next, the loaded and verifiedpattern is read into the phase lock loops, with this pattern again readback to be verified. Now the system clock is started using the phaselock loop output as input. At the completion of IML, the system isdegraded to its slowest clock state and upgraded back to its normalstate with the required number of small incremental steps to the phaselock loops. This insures that all necessary patterns can be loaded intothe phase lock loops without system error. This process takes a fractionof a second to complete on every server that is IMLed.

The pattern to be loaded for speed adjustment purposes such as whengoing from one cooling state to another is generated by a set of digitalI/O lines controlled by the FGAs DIO engines, which is a part of thecage controller (cc) hardware. The FGAs DIO engines are digital I/Olines controlled by cage controller code that interface to the PLLs thatcontrol the system oscillators 263. They are CSA code driven which isrunning on the PU Book 0 cage controller (cc). Before changing the PLLpattern due to a change in cooling state, the existing pattern ismonitored to make sure the adjusting processes were not interrupted, bysaving the line settings of the current pattern.

The CSA code 220 issues a warning service reference code (SRC) to theoperator whenever the CSA code leaves normal clock speed. When theservice is completed, the PCC 210 removes the error states andinterrupts the CSA code 220. The CSA code 220 removes SRC once notified.

The CSA code 220 monitors the actual speeds used for an IML to assurethese speeds are never increased in actual operation even though thecooling state later permits the increased speed. The reason for this isthat the initialization of “Elastic Interfaces” (EI) done during IMLallows only for speed reduction and its clearing, not faster speeds thanthose present during IML initialization and self-tests.

Hence, the CSA code 220 notifies the operator that re-ILM should beavoided while a cooling failure service register is flagged so that whenthe cooling hardware problem is repaired, the server can return to itsfast normal speed without needing a subsequent re-IML. Also contemplatedis a repair and verify procedure that verifies that the clocks havereturned to full speed while a customer engineer is present.

As a further enhancement on the above-described cooling system, apolytetrafluorethylene coating is employed on selected pressure dropareas of expansion structures within the vapor compression refrigerationsystem.

As noted, it has been discovered that material can agglomerate incertain pressure drop areas of the expansion structures within therefrigeration system. During refrigerant/oil transport, certainimpurities and chemically reacted byproducts may come out of solution inthe pressure drop areas as the refrigerant cools down. By way ofexample, FIGS. 4 & 5 depict part of an expansion valve, generallydenoted 400, which includes a first element 410 having an expansionorifice 430, and a second element 420 having a tapered expansion pin440. As shown, the expansion pin 440 controls the amount of refrigerantpassing through expansion orifice 430, where refrigerant is assumed toflow left-to-right in the drawings illustrated. For the coolingapplications described hereinabove, the expansion pin 440 is steppedopen in very small increments to allow controlled flow of refrigerantthrough expansion orifice 430 into a pressure drop area defined betweenopposing surfaces 450 of elements 410 & 420

During refrigerant/oil transport through a hot compressor, anylong-chain molecules and other typically non-soluble compounds at roomtemperature can go into solution in the hot mixture. These, as well asother physically transported impurities, then fall out of the solutionwhen the refrigerant/oil cools down, for example, in the pressure dropareas of the expansion structure. A layer of “waxy” material can buildup in the pressure drop areas and act as a sticky substance which thencatches other impurities. FIG. 6 depicts one example of an expansion pin440 wherein contaminant material 460 has amassed in certain pressuredrop areas of surfaces of the pin exposed to the refrigerant flow path.This amassing of material can interfere with the normal control volumesand interfere with the control of motor steps (e.g., due tounpredictable vavle characteristic changes). This is particularly truein a vapor compression refrigeration system employed as described abovesince the control of the expansion valves in this implementation is verysensitive and refrigerant expansion structure geometries are typicallyvery small. Experimentation has shown that cleaning contaminant materialfrom the pressure drop areas of expansion valves will typically fix anyvalve control problem resulting therefrom.

Thus, the solution presented herein is to apply a polytetrafluorethylenecoating to at least portions of one or more expansion structures withinthe vapor compression refrigeration system in the pressure drop areas ofthe expansion structures. For example, FIG. 7 depicts apolytetrafluorethylene coating 770 over an expansion pin 700 of anexpansion valve to be disposed within the vapor compressionrefrigeration system. In FIG. 8, the polytetrafluorethylene coating isshown also disposed on the inner surface of element 710 definingexpansion orifice 730 in the pressure drop area of the expansion valvedefined between the opposing surfaces 750 of element 710 and element720, that is, the area which contains the tapered expansion pin 740 asshown. The polytetrafluorethylene coating can be applied to the exposedsurfaces of a refrigerant expansion structure in the pressure drop areaemploying any conventional technique, such as vapor deposition. Thepolytetrafluorethylene coating has a thickness sufficient to inhibit theaccumulation of material in any pressure drop area without changing apressure drop characteristic of the pressure drop area. For example, ifthe expansion orifice is 30 mils in diameter, then the thickness of thepolytetrafluorethylene coating may be 5 microns or less. Again, the goalof applying a polytetrafluorethylene coating is to make the exposedsurfaces sufficiently slippery in the pressure drop areas of theexpansion structures to inhibit the agglomeration of material onto thosesurfaces. This goal is achieved by the combination of refrigerant forcethrough the pressure drop area and the surface energy properties of thepolytetrafluorethylene, which together will reduce or eliminatecontaminants from agglomerating.

Although preferred embodiments have been depicted and described indetail herein, it will be apparent to those skilled in the relevant artthat various modifications, additions, substitutions and the like can bemade without departing from the spirit of the invention and these aretherefore considered to be within the scope of the invention as definedin the following claims.

1. A cooling system for cooling at least one heat generating electronicscomponent, the cooling system comprising: a vapor compressionrefrigeration system, the vapor compression refrigeration systemcomprising a condenser, at least one expansion structure, at least oneevaporator, and a compressor coupled in fluid communication to define arefrigerant flow path and allow the flow of refrigerant therethrough;and wherein the at least one evaporator facilitates removal of heatproduced by the at least one heat generating electronics component, andwherein at least a portion of the at least one expansion structure iscoated with a polytetrafluorethylene in the refrigerant flow path forinhibiting accumulation of material thereon.
 2. The cooling system ofclaim 1, wherein the at least a portion of the at least one expansionstructure comprises a pressure drop area of the at least one expansionstructure.
 3. The cooling system of claim 2, wherein the vaporcompression refrigeration system comprises multiple expansion structurescoupled in the refrigeration path, each expansion structure comprising apressure drop area coated with a polytetrafluorethylene in therefrigerant flow path.
 4. The cooling system of claim 2, wherein thepolytetrafluorethylene coating has a thickness sufficient to inhibitaccumulation of material in the pressure drop area without changing apressure drop characteristic of the pressure drop area.
 5. The coolingsystem of claim 1, wherein the at least one expansion structurecomprises an expansion valve including an expansion pin and an expansionorifice defining a pressure drop area, and wherein the pressure droparea is coated with a polytetrafluorethylene in the refrigerant flowpath.
 6. The cooling system of claim 5, wherein the expansion valve isan electronic expansion valve.
 7. A vapor compression refrigerationcooling system for cooling at least one heat generating electronicscomponent, the cooling system comprising: a condenser; a firstelectrically controlled expansion valve coupled to the condenser; afirst evaporator coupled to the first electrically controlled expansionvalve; a second electrically controlled expansion valve coupled to thecondenser; a second evaporator coupled to the second electricallycontrolled expansion valve; a controller providing control signals tothe first electrically controlled expansion valve and the secondelectrically controlled expansion valve to control operation of thefirst electrically controlled expansion valve and the secondelectrically controlled expansion valve; a compressor coupled to thefirst evaporator, the second evaporator and the condenser; and whereinthe condenser, the first electrically controlled expansion valve, thefirst evaporator, the second electrically controlled expansion valve,the second evaporator, and the compressor are coupled in fluidcommunication to define multiple refrigerant flow paths, eachrefrigerant flow path allowing the flow of refrigerant therethrough, andwherein the first evaporator and the second evaporator facilitateremoval of heat produced by the at least one heat generating electronicscomponent, and wherein at least a portion of the first electricallycontrolled expansion valve and at least a portion of the secondelectrically controlled expansion valve are coated with apolytetrafluorethylene in respective refrigerant flow paths forinhibiting accumulation of material thereon.
 8. The cooling system ofclaim 7, wherein the at least a portion of the first electricallycontrolled expansion valve comprises a pressure drop area of the firstelectrically controlled expansion valve, and wherein the at least aportion of the second electrically controlled expansion valve comprisesa pressure drop area of the second electrically controlled expansionvalve.
 9. The cooling system of claim 8, wherein the pressure drop areascomprise areas where refrigerant expansion occurs during a vaporcompression cycle of the vapor compression refrigeration system.
 10. Thecooling system of claim 8, wherein the polytetrafluorethylene coatinghas a thickness sufficient to inhibit accumulation of material in thepressure drop areas without changing pressure drop characteristics ofthe pressure drop areas.
 11. The cooling system of claim 7, wherein thefirst electrically controlled expansion valve comprises a firstexpansion pin and a first expansion orifice defining a first pressuredrop area, and wherein the second electrically controlled expansionvalve comprises a second expansion pin and a second expansion orificedefining a second pressure drop area, and wherein the first pressuredrop area and the second pressure drop area are coated with apolytetrafluorethylene in the refrigerant flow path.
 12. The coolingsystem of claim 7, wherein the cooling system is for cooling multipleheat generating electronics components, and wherein the first evaporatorfacilitates removal of heat produced by a first electronics component ofthe multiple heat generating electronics components and the secondevaporator facilitates removal of heat produced by a second electronicscomponent of the multiple heat generating electronics components.
 13. Amethod of fabricating a vapor compression refrigeration system forcooling at least one heat generating electronics component, the methodcomprising: (i) providing a condenser, at least one expansion structure,at least one evaporator, and a compressor; (ii) providing apolytetrafluorethylene coating on at least a portion of the at least oneexpansion structure; (iii) coupling the condenser, at least oneexpansion structure, at least one evaporator and compressor in fluidcommunication to define a refrigerant flow path; and (iv) providingrefrigerant within the refrigerant flow path of the vapor compressionrefrigeration system to allow for cooling of the at least one heatgenerating electronics component employing sequential vapor compressioncycles, wherein the polytetrafluorethylene coating is provided on the atleast a portion of the at least one expansion structure in therefrigerant flow path for inhibiting the accumulation of materialthereon.
 14. The method of claim 13, wherein the providing (ii)comprises providing the polytetrafluorethylene coating on a pressuredrop area of the at least one expansion structure.
 15. The method ofclaim 14, wherein the providing (i) comprising providing multipleexpansion structures, and wherein the coupling (iii) comprises couplingthe multiple expansion structures in the refrigerant flow path, eachexpansion structure comprising a pressure drop area coated with apolytetrafluorethylene in the refrigerant flow path.
 16. The method ofclaim 14, wherein the providing (ii) comprises providing thepolytetrafluorethylene coating with a thickness sufficient to inhibitaccumulation of material in the pressure drop area without changing apressure drop characteristic of the pressure drop area.
 17. The methodof claim 13, wherein the providing (i) comprises providing an expansionvalve as the at least one expansion structure, the expansion valveincluding an expansion pin and an expansion orifice defining a pressuredrop area, and wherein the providing (ii) comprises providing thepolytetrafluorethylene coating in the pressure drop area in therefrigerant flow path.
 18. The method of claim 17, wherein the providing(i) comprises providing an electronic expansion valve as the expansionvalve.